1. Field of the Invention
The invention relates to communication systems. More particularly, the invention relates to time tracking in a wireless communication system.
2. Description of the Related Art
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10. FIG. 1 shows the three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows a portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote units may be any type of communication unit. For example, the remote units can be hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14.
In a typical wireless communication system, such as that illustrated in FIG. 1, some base stations have multiple sectors. A multi-sectored base stations comprises multiple independent transmit and receive antennas as well as independent processing circuitry. The principles discussed herein apply equally to each sector of a multi-sectored base station and to a single-sectored independent base station. For the remainder of this description, therefore, the term "base station" can be assumed to refer to either a sector of a multi-sectored base station, a single-sectored base station or a multi-sectored base station.
In a code division multiple access (CDMA) system, remote units use a common frequency band for communication with all base stations in the system. Use of a common frequency band adds flexibility and provides many advantages to the system. For example, the use of a common frequency band and enables a remote unit to simultaneously receive communications from more than one base station as well as transmit a signal for reception by more than one base station. The remote unit can discriminate and separately receive the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties. Likewise, the base station can discriminate and separately receive signals from a plurality of remote units. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
CDMA communication techniques offer many advantages over narrow band modulation techniques. In particular, the terrestrial channel poses special problems by the generation of multipath signals which can be overcome through the use of CDMA techniques. For example, at the base station receiver, separate multipath instances from a common remote unit signal can be discriminated and separately received using similar CDMA techniques as those used to discriminate between signals from the various remote units.
In the terrestrial channel, multipath is created by reflection of signals from obstacles in the environment, such as trees, buildings, cars and people. In general, the terrestrial channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over a multipath channel, a stream of pulses is received. In a time varying multipath channel, the received stream of pulses changes in time location, amplitude and phase as a function of the time at which the ideal impulse is transmitted.
FIG. 2 shows an exemplifying set of signal instances from a single remote unit arriving at the base station. The vertical axis represents the power received on a dB scale. The horizontal axis represents the delay in arrival of the instances at the base station due to transmission path delays. An axis (not shown) going into the page represents a segment of time. Each signal instance in the common plane of the page has arrived at a common time but was transmitted by the remote unit at a different time. In a common plane, peaks to the right represent signal instances which were transmitted at an earlier time by the remote unit than peaks to the left. For example, the left-most peak 20 corresponds to the most recently transmitted signal instance. Each signal peak 20-30 corresponds to a signal which has traveled a different path and, therefore, exhibits a different time delay and a different phase and amplitude response.
The six different signal spikes represented by peaks 20-30 are representative of a severe multipath environment. Typical urban environments produce fewer usable instances. The noise floor of the system is represented by the peaks and dips having lower energy levels.
Note that each of the multipath peaks varies in amplitude as a function of time as shown by the uneven ridge of each multipath peak 20-30. In the limited time shown, there are no major changes in the amplitude of the multipath peaks 20-30. However, over a more extended time range, multipath peaks diminish in amplitude and new paths are created as time progresses. The peaks can also slide to earlier or later time offsets as path distances change due to movement of objects in the coverage area of the base station.
In addition to the terrestrial environment, multiple signal instances can also result from the use of satellite systems. For example, in a GlobalStar system, remote units communicate through a series of satellites rather than terrestrial base stations. The satellites orbit the earth in approximately 2 hours. The movement of the satellite through its orbit causes the path distance between the remote unit and the satellite to change over time. In addition, as a satellite moves out of range of the remote unit, a soft hand-off from one satellite to another satellite is performed. During the soft hand-off, the remote unit demodulates signals from more than one satellite. These multiple signal instances can be combined in the same manner as the multipath signal instances in a terrestrial system. One difference, however, is that the signal instances tend to be offset from one another by approximately 0-500 microseconds in the terrestrial environment while the signal instances received through two satellites tend to be offset from one another on the order of 0-20 milliseconds.
In both terrestrial and satellite systems, usable signal instance can result from other sources. For example, in order to overcome the effects of fading, often two or more diversity receivers are used. In addition, multiple signal instances are created during softer hand-off between sectors of a common base station.
FIG. 3 is a block diagram of a prior art rake receiver. The rake receiver shown in FIG. 3 comprises N demodulation elements 100A-100N. The demodulation element 100A has been detailed in FIG. 3 and the demodulation elements 100B-100N can be assumed to be configured in a similar manner to the demodulation element 100A. Incoming signal samples are coupled to the input of each of the demodulation elements 100A-100N. Within the demodulation element 100A, a despreader 102 correlates the incoming signal samples with the spreading code used to spread the signal at the corresponding remote unit. The output of the despreader 102 is coupled to a Fast Hadamard Transformer (FHT) 104. The FHT 104 is configured to correlate the despread samples with each one of a set of possible symbol values. For example, in one embodiment, the system operates in accordance with the Telephone Industry Association, Electronic Industry Association (TIA/EIA) interim standard entitled "Mobile Station--Base Station Compatibility Standard For Dual-Mode Wideband Spread Spectrum Cellular System," TIA/EIA/IS-95, generally referred to as IS-95, the contents of which are incorporated by reference herein in their entirety. In such a system, a group of 6 data bits are mapped into 1 of 64 orthogonal Walsh symbols. The FHT 104 correlates the despread samples with the 64 orthogonal Walsh symbols. The FHT 104 produces a set of different voltage levels, one voltage level corresponding to each of the possible symbol values.
The output of the FHT 104 is coupled to an energy determination block 106 which determines a corresponding energy value for the each of the possible symbol values. The output of the energy determination block 106 is coupled to a multipath combiner 110. In addition, the energy output values of the demodulation elements 100B-100N are also coupled to the input of the multipath combiner 110. The multipath combiner 110 combines the energy values output by each of the demodulation elements 100A-100N on a symbol by symbol basis to determine a combined set of energy values, one energy value corresponding to each of the possible symbol values.
The output of the multipath combiner 110 is coupled to a maximum detector 112. The maximum detector 112 determines the most likely transmitted data value based upon the combined set of energy values. For example, in one embodiment, the maximum detector 112 operates in accordance with U.S. Pat. No. 5,442,627 entitled "Non-Coherent Receiver Employing a Dual-Maxima Metric Generation Process" assigned to the assignee hereof and incorporated herein in its entirety by this reference. The output of the maximum detector 112 is coupled to digital processing circuitry which executes further digital processing.
As noted above, each demodulation element 100A-100N tracks in time the signal instance to which it is assigned. In order to do this, the demodulation element 100A demodulates the incoming signal samples at an earlier and a later time offset than the nominal on-time offset. By comparing the energy results of the early and late process, the accuracy of the current on-time estimate can be determined according to well-known principles of communication. As shown in FIG. 3, an early despreader 110A despreads the incoming signal samples with a time offset advanced by approximately one half chip from the time offset used by the despreader 102. Likewise, a late despreader 110B despreads the incoming signal samples with a time offset retarded by approximately one half chip from the time offset used by the despreader 102. The voltage levels output by the early despreader 110A and the late despreader 110B are stored temporarily in a buffer 112A and a buffer 112B, respectively.
As shown in FIG. 3, the output of the energy determination block 106 is also coupled to a maximum detector 108 which determines the most likely transmitted data value based upon the output of the energy determination block 106. A symbol decovering block 114A correlates the despread samples stored in the buffer 112A with the symbol corresponding to the most likely transmitted data value. For example, in one embodiment, a Walsh symbol corresponding to the most likely transmitted data value is correlated with the stored samples in a similar manner in which the incoming signal samples are correlated with the spreading code within the early despreader 110A. In a similar manner, a symbol decovering block 114B correlates the despread samples stored in the buffer 112B with the symbol corresponding to the most likely transmitted data value. The symbol decovering blocks 114A and 114B produce an early energy value and a late energy value, respectively.
The early and late energy values are stored in a gate and compare block 116. If the most likely transmitted data value chosen by the maximum detector 112 is the same as the data value generated by the maximum detector 108, the block 116 compares the early and late energy levels. According to well-known principles of communication theory, if the two values are equal, the proper time offset is being used by the despreader 102. However, if one value is larger than the other, the time offset used by the despreader 102 is offset from the ideal time offset. A time trackor 118 accumulates the energy value output by the gate and compare block 116 to determine an updated time offset value for use by the despreader 102. In addition, the time offset output of the time trackor 118 is typically forwarded to a system controller 120 which performs a demodulation element assignment algorithm.
If the data value generated by the maximum detector 112 is different than the data value generated by the maximum detector 108, it is assumed that the maximum detector 108 has made an error. This assumption is based upon the well-known principle of communication theory that by combining the energy levels produced by several demodulation elements, a more accurate determination of the most likely transmitted data value is made. For this reason, on average, the maximum detector 112 produces a more accurate estimate of the transmitted data than the maximum detector 108. Therefore, if the data value generated by the maximum detector 108 is not the same as the value generated by the maximum detector 112, the corresponding early and late energy values are likely to have been determined using an erroneous data value and, thus, do not present viable data. For this reason, the gate and compare block 116 discards these values and does not forward them to the time trackor 118.
Additional information concerning rake receivers, demodulators and time tracking can be found in U.S. Pat. No. 5,654,979 entitled "Cell Site Demodulation Architecture for a Spread Spectrum Multiple Access Communication", U.S. Pat. No. 5,644,591 "Method and Apparatus for Performing Search Acquisition in a CDMA Communications system", U.S. Pat. No. 5,561,618 entitled "Method and Apparatus for Performing a Fast Hadamard Transform", U.S. Pat. No. 5,490,165 entitled "Demodulation Element Assignment in a System Capable of Receiving Multiple Signals", U.S. Pat. No. 5,805,648 entitled "Method and Apparatus for Performing Search Acquisition in a CDMA Communication System", each of which is assigned to the assignee hereof and incorporated in its entirety herein by this reference.
One deficiency of such operation is that a substantial amount of invalid time tracking data is developed and then discarded by the gating process. For example, even if the error rate of the demodulation elements is individually as high as eighty percent, the error rate of the combined signal can be as low as ten percent and, thus, present a viable communications channel. Thus, if the error rate of the demodulation element is about eighty percent, about 4 out of 5 energy values calculated by the symbol decovery blocks 114A and 114B are invalid and, thus, discarded by the gating process and not used by the time trackor 118. In such a situation, the time trackor operates based upon only about twenty percent of the available energy. Such operation unnecessarily delays the time tracking process and also decreases the precision of the time tracking process.
When the time offset used by the despreader 102 is offset from the ideal offset, the energy output by the energy determination block 106 is reduced. FIG. 4A is a graph showing the energy received as a function of the time offset used to demodulate a signal. In FIG. 4A, the vertical axis represents the energy detected by the rake receiver and the horizontal axis represents the time offset used by the rake receiver to demodulate the signal. When the rake receiver demodulates the signal with ideal synchronization at an ideal on-time alignment t.sub.0, the rake receiver detects the maximum energy available from the signal as shown by data point 122 on FIG. 4A. If the rake receiver demodulates the remote unit signal using a timing which is delayed by a time offset 6, from the ideal on-time alignment to to a late time alignment, t.sub.1, the rake receiver detects less energy as shown by data point 124 on FIG. 4A. In a like manner, if the rake receiver demodulates the remote unit signal using a timing which is advanced by the time offset .delta..sub.t from the ideal on-time alignment t.sub.0 to an early time alignment t.sub.e, the rake receiver detects less energy as shown by data point 123 in FIG. 4A. So long as the early and late alignments are offset from the on-time alignment by the same amount of time and the on-time alignment is ideal, the energy detected at the early and late alignments is the same.
FIG. 4B is a similar diagram to FIG. 4A except that an on-time alignment t.sub.0 ' has been skewed to be slightly late of the ideal timing. Notice that due to the offset, the amount of energy detected at data point 126 is less than that detected in the ideal case at data point 122. If the rake receiver demodulates the signal at time offset .delta..sub.t earlier than the on-time alignment t.sub.0 ' at the early time alignment t.sub.c ' in FIG. 4B as shown by data point 127, the rake receiver detects more energy than at data points 62 and 64 of FIG. 4A. Likewise, if the rake receiver demodulates the remote unit signal at an offset delayed by time offset .delta..sub.t from the on-time alignment t.sub.0 ' at the late time alignment t.sub.1 ' as shown by data point 128, the rake receiver detects less energy than at data points 62 and 64 in FIG. 4A and also data point 127 in FIG. 4B. By comparing the energy detected by the rake receiver at an early time alignment and a late time alignment, it is possible to determine whether the on-time alignment is ideally aligned. If the early and late time alignments yield the same energy level, the rake receiver is likely to be detecting the signal with an accurate time alignment. If an energy level detected at the early alignment is significantly higher than the energy level detected at the late alignment, the rake receiver is likely to be detecting the signal with an alignment delayed from the ideal. If an energy level detected at the late alignment is significantly higher than the energy level detected at the early alignment, the rake receiver is likely to be detecting the signal with an alignment advanced from the ideal.
The reduction in energy produces a corresponding reduction in the total energy produced by the multipath combiner 110. According, the lower total energy produces a corresponding decrease the accuracy of the data value determination process executed by the maximum detector block 112, thus reducing the overall performance of the receiver. In addition, the reduction in energy produces less accurate time tracking for weak signal instances than for strong signal instances, thus further reducing the usable energy produced by weak signal instances.
Therefore, there has been a long-felt need in the art for a more accurate system and method of time tracking.